Current generation of SiC/SiC ceramic matrix composites (CMC) rely almost entirely on the silicon carbide (SiC) fibers to carry the load above the matrix cracking stress. As a result, the high temperature usefulness of these CMCs falls well below their theoretical capabilities.
Currently, there are four broad classes of silicon carbide-based CMCs depending on the processing methods: (a) Melt infiltration (MI), (b) Chemical Vapor Infiltration (CVI), (c) Polymer Infiltration and Pyrolysis (PIP), and (d) Hot Pressing (HP). In all CMC manufacturing, the starting material is a 1-D SiC fiber tow, a 2-D woven cloth, or 3-D woven fiber preforms. The starting material is then coated with single or multi-layered interface coatings to reduce strength degradation of the fibers during composite fabrication, as well as to provide crack deflection path along the interface during loading of the composite.
Depending on the consolidation methods, the interface coated 1-D SiC fiber tows wound on a drum coated with the matrix slurry or the ceramic yielding polymer, dried, cut into required size, stacked, pressed to prepare a green body or the interface coated 2-D woven cloth are further coated with matrix slurry or ceramic yielding polymers, dried, cut into required size, stacked and pressed to prepare a green body. The green body can be hot pressed directly or can be pyrolyzed then wicked with molten silicon or ceramic yielding polymer. Also, 2-D cloth or 3-D fiber preforms are compressed in a tooling graphite die, interface coated, and then infiltrated with SiC coating by CVI to stiffen the fibers, as well as prepare composite preforms with certain levels of interconnected open porosity. These composite preforms can be infiltrated with ceramic particles and then with silicon or ceramic yielding polymer.
Irrespective of the manufacturing details, the final step in the melt infiltration (MI) fabrication of many CMCs generally involves infiltrating or wicking molten silicon into particulate filled or unfilled SiC composite preforms. Although these CMCs are denser than those prepared by non-MI methods, the presence of free silicon in the matrix restricts their use to below 1623 Kelvin (K) because of silicon diffusing into interface coating(s) and the load bearing SiC fibers, and drastically degrading the in-plane properties.
In the case of CMCs fabricated by CVI, the presence of angular voids in the CVI matrix significantly reduces the matrix cracking stress and through-the-thickness thermal conductivity. Additionally, oxygen ingression from the external surfaces through surface-connected voids and cracks in the matrix can lead to the oxidation of the protective fiber coatings, thereby reducing durability of the CMCs.
Thus, the development of high temperature, lightweight, SiC fiber-reinforced ceramic composites with engineered matrices (EM), which exhibits a degree of matrix plasticity and self-healing characteristics, would be a significant improvement over the current state of the art.